(CNN) -- Iberia ground and cabin crews Monday began what they said was the biggest strike in the airline's history, but the company countered that it had already re-assigned most passengers to other flights to reduce airport chaos.
Five people were arrested during the strike for going beyond a police line at Madrid's Barajas airport, a government spokesman told CNN.
Earlier, Iberia said in a statement that it had found other flights for 60,000 clients and will give ticket refunds to another 10,000.
But Iberia canceled 415 flights this week, about 40% of its total, and three smaller airlines for which Iberia provides ground services canceled an additional 800 flights.
Most routes -- within Spain, or to nearby destinations in Europe or much longer distances -- were operating, but in many cases with fewer than the normal daily number of flights.
The unions are protesting layoffs, salary cuts for many of the remaining staff and company downsizing that has already seen elimination of flights from Madrid to Athens, El Cairo and Istanbul. In April, Iberia will also stop flying from Madrid to Havana, Santo Domingo, Montevideo and San Juan de Puerto Rico.
Iberia has announced 3,800 layoffs, about 19% of the staff, part of an effort to reduce losses, which were more about $350 million last year.
Ground crews and in-flight attendants announced 15 days of strikes, during three weeks Monday to Friday, starting Monday. Pilots are not on strike this week but are due to join in for next strikes, for the weeks of March 4 and March 18.
Union leaders call it the biggest strike in Iberia's history because it's the first time that all three units -- pilots, cabin crews and ground personnel -- have joined forces to strike and it's the first time so many days of strikes have been announced at one time.
Iberia called the situation "totally normal" on Monday, given the strike. But Manuel Atienza, of the General Workers Union (UGT) said the strike will have more impact as it continues, because overloaded ground crews, even while complying with legally-mandated minimum services, will fall farther behind on tasks like baggage handling.
Unions say that Iberia, which merged with British Airways in 2011, has been suffering ever since as the junior partner, while the majority shareholder British has expanded.
Iberia pilots last year went on strike when Iberia created a low-cost carrier called Iberia Express. It and two other airlines that get ground services from Iberia - Air Nostrum and Vueling - also canceled fights this week.